Method of etching and a photographic gelatin resist forming element therefor



United States Patent 3,488,191 METHOD OF ETCHING AND A PHOTOGRAPHIC GELATIN RESIST FORMING ELEMENT THEREFOR Thomas J. ODonnell, Clayton, NJ. (63 S. Peru St., Plattsburgh, NY. 12901) No Drawing. Filed Jan. 26, 1966, Ser. No. 523,020 Int. Cl. G03c 5/00, 1/90 US. Cl. 96-36 9 Claims ABSTRACT OF THE DISCLOSURE A resist forming element by means of which a previously formed gelatin stencil can be transferred to a nonporous, non-absorptive material to be etched in combination with a water vapor pervious support that can be detached by the action of an ingredient of the etchant.

This invention relates to the art of etching generally and more particularly of etching metal plates, cylinders and other members for use in letter press and offset printing, chemical milling and the like.

Heretofore, metal etching of letter press and offset printing plates required the coating of a resist, generally a bichromated colloid, on the metal uiformly by means of a whirler, the use of a vacuum frame to hold a negative in place and insure intimate contact against the resist and the exposure of the material using high intensity arcs before final processing with an etchant. The primary object of the present invention is to provide a special photographic element which simplifies the conventional etching process and effects important economies by eliminating the need for a whir'ler or other means to uniformly coat the metal with the resist, eliminating the need for a separate negative and hence the vacuum frame or other means to hold it tightly against the resist on the metal and the high intensity arcs to expose the resist through the negative while it is held to the metal.

An important object of the invention is to provide a stencil for and method of etching materials particularly metals using the same which enables small photographers to make halftones for weekly newspapers, school publications and the like with speed and efficiency without employing special costly equipment and under any condition of temperature and humidity in which photographic processing is feasible.

A further object of the invention is to provide a metal etching process which eliminates the need for the silk screening of resists on the metal or coating with a bichromated colloid as now required in chemical milling, i.e. the production of printed circuits, nameplates and other miscellaneous metal forms.

Other objects and features of the invention will become more apparent as the description of the details of the invention proceeds.

The invention consists essentially of three components. The first is a fully formed stencil which is derived from an element comprising a support, a resin and a gelatinsilver halide emulsion thereon which has been photographically processed. The second is an adhesive to adhere the photographic element to the "material to be etched. The third is an etchant incorporating a chemical which serves as a diluent for the etchant and to which the gelatin is impervious and which acts as a solvent for the resin to permit stripping of the support, whereby the stencil thus formed is bonded to the subject material, etching continuing to the desired depth.

The element serving as the precursor of the stencil comprises a water vapor pervious support or backer, a photographic gelatin-silver halide emulsion and a resin layer therebetween which is exposed, developed and subjected to a gelatin etchant to produce a positive relief. An additional substrate of gelatin may be interposed between the resin and emulsion to bond the latter more firmly to the former. The ratio of gelatin to silver halide salts in the emulsion is such that the gelatin etchant, preferably peroxide, is rendered very effective and the opacity of the emulsion is decreased to afford greater light penetration.

The water vapor pervious support is preferably made of paper not more than 3 mils in thickness to permit efficient thorough drying of the subsequent stencil when adhered to metal. The paper stock is dipped into a resin solution of viscosity sufficient to yield a desired thickness of resin on the surface of the paper when dried. The resin solution consists of a cellulose derivative, such as the acetate or nitrate or ethyl cellulose, dissolved in a suitable solvent such as acetone to which a plasticizer may be added. The viscosity of the resin solution is such that when the paper stock is dipped into it and then air dried, the layer of resin on the paper surface has a thickness in the order of magnitude of 1 micron, while the resin which is absorbed in the paper does not exceed about 4 microns in thickness. Excessive thickness or overall bulk of the resin not only increases drying time but also magnifies flaws caused when foreign particles become accidentally embedded therein when the stencil is subsequently adhered to metal and etched. The maximum overall thickness of the resin should not exceed 5 microns.

The paper coated with the resin and dried is then dipped into an aqueous emulsion of pre-hardened gelatin and silver halide salts, such as the iodide, the bromide or mixtures thereof. Here again the viscosity of the emulsion is such that when the element is removed from the bath and air dried it should be about 5 microns in thickness, i.e. thin enough to permit air drying in about onehalf hour when the subsequent stencil is adhered to metal and thick enough to serve as a resist. The ratio by weight of the gelatin to the silver salts must be at least 2:1 to reduce opacity and permit formation of small shadow dots completely penetrating the emulsion in the negative image upon first exposure to the positive original.

It has been found that firmer bonding of the emulsion to the resin can be attained by coating a substrate of prehardened gelatin on the dried resin layer of a thickness in the order of magnitude of about 1 micron. This, of course, can be effected by dipping the support with the dried resin coating thereon in an aqueous gelatin solution and then air drying the unit before dipping it in the aforementioned gelatin-silver halide emulsion.

In use, the photographc element consisting as it does of the paper support, the intermediate layer of dried resin thereon and the outer layer of dried gelatin-silver halide emulsion thereover (with or without the substrate of gelatin intermediate the resin and outer layer of sensitive emulsion) is placed in a camera behind a halftone screen, preferably lines and using a prism to obtain lateral reversal if required, to expose the emulsion until the desired size highlight is obtained. Then it is flash exposed, i.e. to a white surface or a dim light, to produce dots in the shadow areas. Exposure and flash should be regulated to yield 40% dot in the lightest areas and a pinpoint dot in the blacks of the subsequent positive. The shadow dots should completely penetrate the emulsion.

The element is then subjected to a conventional contrast developer such as hydroquinone, paraformaldehyde, etc., at a temperature of 65 F.100 F., preferably 3 mins. at F., rinsed in an acid stop bath, generally 3% acetic acid, and finally rinsed with water for several minutes thereby producing a negative silver image in the emulsion.

The developed element is then immersed in a peroxide etchant bath to remove the silver image and gelatin in situ only of the emulsion and leave the gelatin with the unreduced silver salts intact. This converts the negative halftone image directly into a positive relief image. It has been found that equal parts by volume of the following aqueous solutions produces an effective gelatin etchant which can be employed at 65 F. to 100 F.:

A. Copper sulfate gms 100 Sodium chloride gms l Acetic acid (glacial) cc 150 Water to cc 1000 B. Hydrogen peroxide percent 3 Other copper salts, such as copper nitrate, can be substituted for the copper sulfate and other alkali salts, such as potassium bromide, can be substituted for the sodium chloride. After removal of the element from the etch bath it is brushed lightly to completely remove the softened gelatin.

The subject material to be ultimately etched is then prepared for receipt of the second or adhesive component by cleaning the surface thereof. In the case of metal, it is prepared by washing it with a cleaning powder to remove oils, rinsed with water, immersed in a 3% acetic acid and finally rinsed with water. A pool of the adhesive component is placed on the subject material and the positive relief is firmly squeegeed to the subject material with the relief surface facing the material. If the relief image was previously dried, it should be immersed in water at a temperature of about 110 F. for about 2 minute-s before it is squeegeed over the adhesive onto the subject material.

In the case of metals, such as zinc, the adhesive serves the dual function of causing the relief image to adhere firmly to the metal and of controlling the action of the metal etchant. Thus, the dry adhesive which accumulates between the fine detail and halftone highlight dots controls the action of the metal etchant by restraining the, formation of gas bubbles during etching and preventing them from clinging tenaciously to the metal in which condition further action of the etchant is retarded. The following adhesive has been found most effective for metals, such as zinc, both as to properties of adhesion and viscosity:

Stock solution:

Gelatin gms 4 Acetic acid (glacial) cc 12 Water cc 400 Methyl alcohol cc 60 Acetone cc 30 The stock solution is diluted with Water in the ratio of 1 part stock solution to 10 parts of water by volume.

After adhesion of the relief image to the metal, the material is thoroughly air dried and immersed in the third component or metal etchant which consists essentially of an inorganic acid or metallic salts, such for example as stannous chloride, as the metal etchant and incorporates a chemical which serves as a diluent for the etchant and to which the gelatin is impervious and which acts as a solvent for the resin. Thus, for a zinc, copper or #303 stainless steel plate, 1 part nitric acid (sp. g. 1.42) and 7 parts acetone by volume is effective. Sulfuric acid may be substituted for the nitric acid and other resin solvents, i.e. solvents for ethyl cellulose, cellulose acetate and cellulose nitrate, can be used such as isopropyl alcohol, methyl alcohol, monomethyl ether of ethylene glycol and ketones other than acetone. For aluminum plates, concentrated hydrochloric acid replaces the nitric acid. The ratio of solvent to acid should not be less than to 1 as this will weaken the resist.

The metal etchant should be mixed shortly before use and discarded thereafter. Sufiicient etchant is used to cover the metal in a tray which is rocked frequently during etching. After about 5 minutes, the paper backing or support an be read y peeled off leav ng h s enc of g a in relief firmly bonded to the metal via the adhesive. Etching is continued until the desired depth of image is obtained. With a 65 line screen, 25 minutes of etching gives a highlight depth of about 4 mils and a shadow dot depth of about 2 /2 mils. During the etch, the 40% starting highlight dot is reduced to a 10% dot and the shadow dots increased in size to yield a 90% tone. As the temperature of the etchant rises, a water jacket may be employed to keep the temperature at about F.

Finally, the metal is rinsed in acetone, for about 5 minutes, dried thoroughly and brushed with a stiff brush to remove overhang. The plate is now ready for use in letter press and olfset printing, as well as chemical milling. In the latter case, the photographic element is exposed by any photographic means to a line original, processed as described above to a relief member which is adhered to a copper clad electrically non-conductive member conventionally used for printed circuits and etched until the copper is removed from the non-working areas.

As stated earlier, the process is applicable to non-metallic materials to be etched as well. The relief image and adhesive. are applied as described hereinbefore except that the etchant is adapted to the subject material. Thus, for glass the etchant consists preferably of 20 parts methyl alcohol and 1 part hydrofluoric acid. For plastics, such as cellulose diacetate or nitrocellulose, the etchant consists preferably of 3 parts acetone and 1 part ethylene glycol monoethyl ether. Light brushing during the etch aidsthe dispersal of the gel formed and the ethylene glycol monoethyl ether acts to reduce the deformation of the plastic and to restrain the action of the acetone.

The above process can be carried out by photographers, of moderate skill and with conventional photographic equipment found in small establishments. Using an ordinary photographic enlarger with an original continuous tone negative in the negative holder, the instant photographic element is placed in a pressure printing frame behind a contact halftone screen, and exposed through the screen in the manner of making a conventional enlargement. The negative is removed and a second flash exposure is made. Any combination of exposure and flash may be used which, as stated hereinbefore, will give a dot formation having a 40% positive dot in the whites and a fully penetrating pinpoint dot in the positive blacks on development- The positive image is reversed to negative by a conventional bichromate bleach, exposure and redevelopment after which it is etched until the silver image and gelatin in situ are removed, then it is adhered to the metal and the metal etched with a solvent for the resin as described hereinbefore.

The instant invention can also be employed in the etching process disclosed in US. Patents Nos. 2,640,763- 2,6;40767.

While preferred embodiments of the invention have been described herein, it will be understood that skilled artisgps may make variations without departing from the spiritof the invention and the scope of the appended claims.

claim:

1. lior use in etching, a fully formed stencil for bonding to a non-porous, non-absorptive material to be etched comprised of a support pervious to water vapor, a layer of water-insoluble resin coated thereon and a pre-hardened gelatin-silver halide photographicemulsion thereon which hasheen exposed and developedto form a silver image whi I completely penetrates the emulsion and from which the, ver image and gelatin in situ have been removed to thereby form a positive gelatin relief image.

2 The article of claim 1 wherein said emulsion consists essentially of at least 2 parts gelatin and 1 part silver halide-by weight.

3;. The article of claim 1- and a substrate of gelatin interposed between said resin coating and said emulsion.

4, A method of etching a subject material comprised f p v d ng a f ly f m d stencil de ived from. an etcment including a support insoluble in water and water vapor permeable, coating the support with a water in soluble, water vapor permeable resin, coating thereon a prehardened gelatin-silver halide emulsion, exposing and developing the element to form a silver image, said silver image completely penetrating the emulsion, removing the silver image and gelatin in situ thereby forming a relief image of the emulsion, adhering the relief image and the element to a subject material by means of a Water soluble adhesive, drying by evaporation through the support and etching the surface of the subject material under the selective control of the relief image.

5. The method of claim 4 and the step of interposing a substrate of gelatin between said resin coating and said emulsion.

6. The method of claim 4 and the step of applying the element to the subject material via a gelatinous adhesive.

7. The method of claim 4 and the steps of subjecting the subject material with the element thereon to an etchant containing an ingredient which is a diluent for the etchant and to which the gelatin is impervious and which acts to dissolve the resin, and finally stripping off the support to leave the stencil bonded to the subject material, etching continuing to the required depth.

8. The method of claim 4 wherein the subject material is a metal.

9. The method of claim 4 wherein the subject material is a plastic.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 12/1950 Great Britain.

OTHER REFERENCES Staehle, Kodak Relief Plate, The Photoengravers Bulletin, November 1964, pp. 76-82.

GEORGES F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner US. Cl. X.R. 

